1. Field of the Invention
The present invention relates to a fluorophosphate glass and more particularly to a fluorophosphate glass suitably used in, for example, a color compensating filter for color VTR cameras.
2. Description of Prior Art
The spectral sensitivity of the pickup tube used in color VTR cameras extends generally from a visible region to a near-infrared region (950 nm). Therefore, unless the near-infrared region is eliminated by the use of a filter and the spectral sensitivity of the pickup tube is simulated to the visual sensitivity of humans, the image obtained has a reddish tone and it is imossible to obtain good color reproduction.
Meanwhile, if the ultraviolet side absorption of the filter used in color VTR cameras extends as far as a visible region, the resulting image is dark. Accordingly, such a filter is required to transmit a light of 400-520 nm as much as possible and to absorb a light of 550-950 nm as much as possible. As such a near-infrared absorption filter, there have conventionally been used CuO-added phosphate glasses.
Phosphate glasses, however, inherently have poor weather resistance. In order to improve the weather resistance of these glasses to a level allowing for their practical use, addition of a relatively large amount of Al.sub.2 O.sub.3 is necessary, as disclosed in, for example, Japanese Patent Publication No. 128943/1987. This raises a temperature at which the glass materials melt in glass production (the temperature is hereinafter referred to as "glass melting temperature"). A higher glass melting temperature reduces copper more easily. Therefore, di-valent copper ion (Cu.sup.2+) in glass which shows absorption for near-infrared region is reduced and converted to mono-valent copper ion (Cu.sup.+) which shows absorption for ultraviolet region; as a result, the glass has a lower transmittance at a visible light region, a higher transmittance at an infrared region and accordingly deteriorated properties. Meanwhile, in order to improve the transmittance, it is necessary to lower the glass melting temperature by, for example, alkali addition and thereby to prevent the reduction of Cu.sup.2+ in glass to Cu.sup.+. This, however, invites further deterioration of the weather resistance of glass. Thus, in the past production of a near-infrared absorption filter using a phosphate glass, the best compromise of two contradictory properties, i.e. transmittance and weather resistance has been seeked so as to allow for the practical use of the produced glass. However, it has been impossible to satisfy both of excellent transmittance and excellent weather resistance.
Hence, the object of the present invention is to provide a glass which fully satisfies the transmittance required for the near-infrared absorption filter used in color VTR cameras and which has excellent weather resistance well allowing for the practical use.
Other objectcs of the present invention will be apparent from the following descriptions.